An air conditioner provided with functions such as cooling, heating and dehumidification is equipped with heat-exchanging fins at its heat exchanger unit. In general, fin blanks for forming these heat-exchanging fins are made of aluminum or aluminum alloy as they are desired to have a light weight, excellent workability and superb thermal conductivity.
When such an air conditioner is operated in cooling mode, the heat exchanger unit is brought on an indoor side thereof to a temperature equal to or lower than the dew point of moisture in the air so that the moisture in the air is caused to condense and adhere on the indoor fins arranged on the heat exchanger unit. The shape which the condensed water takes on fin surfaces is determined depending on the wettability of the fin surfaces with water. On low-wettability fin surfaces, condensed water tends to take the form of substantially hemispherical water droplets and further, to form water bridges between the fins, thereby developing resistance to a flow of air through the heat exchanger unit and hence, impairing the air flowability and further producing a noise. In some instances, condensed water may scatter around to soil the peripheries of the air conditioner.
With a view to overcoming these problems, it has already been practiced to subject fin blanks to hydrophilization treatment on their surfaces. Proposed methods include, for example, use of an inorganic hydrophilizer composed of water glass or the like as a principal component (JP 2-423895 B, JP 3-77440 B); and use of an organic hydrophilizer, such as combined use of polyvinyl alcohol, a specific water-soluble polymer and a crosslinking agent (JP 1-299877 A), use of a polyacrylamide (JP 1-270977 A), use of a copolymer of a particular hydrophilic vinyl monomer (JP 6-306247 A), and use of carboxymethylcellulose polymer, N-methylolacrylamide, polyacrylic acid or a zirconium compound (Japanese Patent No. 2,520,308).
The methods making use of inorganic hydrophilizers can impart excellent hydrophilicity to the surfaces of fin blanks. Nonetheless, they are accompanied by a problem in that dies undergo severe wear upon pressing such fin blanks, because hydrophilic coatings formed on the surfaces of the fin blanks are made of a hard material composed of silica (SiO2) as a principal component. They also involve another problem in that an unpleasant odor is felt in the beginning of cooling, as the hydrophilic coatings have property of readily adsorbing unpleasant odor components from the surrounding environment.
In the methods making use of organic hydrophilizers, on the other hand, the hydrophilizers themselves serve as nutrient sources for microorganisms so that microorganisms tend to grow in treatment baths and on hydrophilic coatings. Prevention of their grow this, therefore, needed. In these methods, rot-proof property is hence imparted to treatment baths as needed. Addition of an antibacterial agent to treatment baths is more or less effective for the prevention of rotting of the treatment baths. Any attempt to impart antibacterial activity to coatings themselves requires addition of the antibacterial agent in a greater amount, leading to a problem that the hydrophilicity of the coatings is impaired. To allow the antibacterial activity of hydrophilic coatings themselves to last longer, it has also been proposed to have an antibacterial substance dissolved out from the surfaces of the hydrophilic coatings. The hydrophilic coatings show antibacterial activity as long as the antibacterial substance dissolves out, but there is a limit to the period during which the antibacterial activity is exhibited.
With a view to imparting antibacterial activity to hydrophilic coatings in an attempt to resolve the above-described problems, it has been proposed to use, as a coating-forming material, chitosan having antibacterial activity and to form a chitosan-containing coating on a surface of a fin blank (JP 7-190676 A). This method requires use of an organic acid or an inorganic acid to form chitosan into an aqueous solution, as chitosan is insoluble in water. However, this acid is washed off with the above-described condensed water, resulting in a problem that the chitosan coatings are deprived of film strength and antibacterial activity. Use of a polymer acid such as polyacrylic acid as the above-mentioned organic acid has also been proposed (JP 11-293149 A). However, these methods which make use of chitosan are all accompanied by a problem that a very unpleasant odor is given off upon and after baking a coating on a surface of a fin blank. Such fin blanks are, therefore, not satisfactory as fin blanks for use in air conditioners.
Most of the above-described conventional methods develop a problem that, unless anti-corrosion coating treatment such as chromate treatment is applied to the surface of a fin blank before the formation of a hydrophilic coating thereon, the hydrophilic coating corrodes aluminum and powdering occurs when used for a long time. Upon surface treatment of a fin blank, anti-corrosion undercoating treatment is hence required. The surface treatment, therefore, has to be conducted in two stages. This is not preferred economically.
Therefore, an object of the present invention is to provide an aqueous composition, which has the following features: an excellent hydrophilic coating, which does not form water droplets, can be formed by applying single-stage treatment, specifically by directly coating the aqueous composition onto an article and drying and curing the thus-coated aqueous composition; the coating has excellent mechanical strength even in a state wet with water (hereinafter the property is referred to a word “waterproof” or “waterproofness”), so that it is excellent in waterproofness and corrosion resistance and also superb in lubricity, can withstand severe forming, and can exhibit sustainable hydrophilicity without being stained with adhered lube oil or the like; and moreover, the coating does not produce an unpleasant odor from the coating-forming material and remains free from occurrence of microorganisms such as mold over a long period. In other word, the coating combines both excellent waterproof strength and hydrophilicity. Another object of the present invention is to provide a process for the surface modification of an article by use of the aqueous composition.